1. Field of the Invention
The present invention relates to a color image forming apparatus such as a copying machine, laser beam printer or the like and, more particularly, to a color image forming apparatus which has intermediate transfer means.
2. Description of the Related Art
With regard to image forming methods to obtain color images, there is a method where a multi-colored toner image is formed by repeating to transfer toner images having different colors from a latent image holding means to an intermediate transfer means color by color, the multi-colored toner image being transferred onto a material to be transferred.
With regard to color image forming apparatus using such intermediate transfer means, an apparatus using a cylindrical drum as the intermediate transfer means is disclosed in Japanese Patent Application Laid-Open No.1-198773 (1989).
Generally, there are various kinds of image forming methods to form a multi-colored image using an intermediate transfer means as the reference described above. However, in the apparatus employing this method, a multi-colored toner image is formed on an intermediate transfer means by repeating to sequentially transfer each of toner images having different colors from a latent image holding means to the intermediate transfer means color by color, the multi-colored toner image being transferred onto a material to be transferred to form a multi-colored image. In performing plural times of transferring images as described above, if the condition of transferring image for the following color is not nearly the same as the condition of the transferring image for the first color, there appears differences in the transfers so that the gradations of color in the final image becomes different from its original.
Further, when an image is formed by superposing toner images, misalignment possibly occurs between each color of the toner images.
The problems in connection with transfer and misalignment are especially important for color image forming apparatus, and become a cause to affect the reproducibity of color and the gradation of color. Therefore, it comes that no desired image can be obtained. This is a significant technical challenge to be overcome.
In order to perform transfer preferably, the intermediate transfer body should be of a proper semi-conductive resistivity. And the distribution of resistivity should be uniform throughout the whole surface without unevenness. When the distribution of resistivity is non-uniform and uneven, a miss-transfer occurs. Therefore, the conductive cylindrical base member of drum-shape in the intermediate transfer body has to be coated with a semi-conductive resistive member. With regard to a coating method, the conductive cylindrical base member of drum-shape is covered with a semi-conductive elastic member of belt-shape. The elastic member has a diameter smaller than that of the cylinder of drum-shape, and is expanded and tightly held on the drum by elastic force. Therein, the belt may be simply adhered onto the drum to be fixed. When the whole surface of the belt is adhered, uniform distribution of resistance cannot be obtained due to bubble formation or unevenness in applying of adhesive agent, which leads to malfunction in transfer. Therefore, the belt is held by contracting force without any interposed material. In this construction, when the belt becomes loose due to environmental change or deterioration with age, a very small air layer existing between the belt and the cylinder of drum-shape becomes a large lump due to attraction of the belt by high voltage during transferring. The air lump makes a local and non-uniform resistivity and probably causes a local transfer fault. Further, there is also a possibility to cause misalignment in color by change in the size of the outer diameter of the drum due to the air layer. Further, it is necessary to decrease the adhesiveness of the surface layer in the intermediate transfer body to which toner adheres. In a case there the surface has adhesiveness, where is possibility to decrease the transfer efficiency and the cleaning efficiency for removing remaining toner after transfer, which causes problems of difference in gradations of color, occurrence of ghost or the like. In the reference described above, the change in environment and deterioration with age are not taken into consideration, and therefore, there is a possibility to cause a serious image fault by occurrence of the problem described above.